Boris Johnson today says that Britain has a “once in a lifetime opportunity” to vote to leave the European Union as a way of securing an entirely new relationship with Brussels based around the single market.

Writing in The Daily Telegraph, Mr Johnson calls for Britain to be “brave” and says that “there is only one way to get the change we need – and that is to vote to go”. He says that “EU history shows that they only really listen to a population when it says No”.

“EU history shows that they only really listen to a population when it says No”

Unlike some of those backing a “Brexit”, Mr Johnson raises the possibility that Britain may not ultimately leave the EU in the event of a “Leave” vote.

He calls for Britain to have a deep and co-operative relationship with the EU “on the lines originally proposed by Winston Churchill: interested, associated, but not absorbed; with Europe – but not comprised”.

His decision will electrify the referendum campaign and came as a major blow to David Cameron just one day after the Prime Minister called the June 23 vote.

It sets the stage for a leadership campaign that looks likely to see Mr Johnson face off against George Osborne, the Chancellor, for the chance to succeed Mr Cameron when he steps down.

“This is a once in a lifetime chance to vote for real change in Britain’s relations with Europe,” Mr Johnson adds. “This is the only opportunity we will ever have to show that we care about self-rule. A vote to Remain will be taken in Brussels as a green light for more federalism, and for the erosion of democracy”

Mr Johnson only informed the Prime Minister of his final decision by text message nine minutes before making a public address declaring that he would back a “Brexit”.

It is understood that he was swayed during talks last week with Michael Gove, the Justice Secretary, who is also backing the campaign to leave the EU.

Mr Johnson and Mr Gove are already being described by insiders as a new axis of power in the Conservative Party to rival Mr Cameron and Mr Osborne.

The two senior Tories are joined by five other Cabinet ministers in backing the out campaign. In the end, it is now expected that 13 junior ministers will back a “Brexit” alongside around 150 Tory MPs.

“This is a once in a lifetime chance to vote for real change in Britain’s relations with Europe,” Mr Johnson adds. “This is the only opportunity we will ever have to show that we care about self-rule. A vote to Remain will be taken in Brussels as a green light for more federalism, and for the erosion of democracy.”

He says that it time “to seek a new relationship, in which we manage to extricate ourselves from most of the supranational elements”.

And he rejects claims made by Mr Cameron and his Cabinet allies about the “risks” of a “Brexit”.

Mr Johnson writes: “We will hear a lot in the coming weeks about the risks of this option; the risk to the economy, the risk to the City of London, and so on; and though those risks cannot be entirely dismissed I think they are likely to be exaggerated. We have heard this kind of thing before, about the decision to opt out of the euro, and the very opposite turned out to be the case.”

In a major blow to Mr Cameron, the Mayor of London rubbished the Prime Minister’s claims that his renegotiated settlement with Brussels has achieved “fundamental reform” of the EU. In his Telegraph article, Mr Johnson suggests that because of the EU’s influence, Mr Cameron and other politicians are “impotent” on key issues like immigration.

“Sometimes the public can see all too plainly the impotence of their own elected politicians – as with immigration,” Mr Johnson writes.

“That enrages them; not so much the numbers as the lack of control. That is what we mean by loss of sovereignty – the inability of people to kick out, at elections, the men and women who control their lives. We are seeing an alienation of the people from the power they should hold, and I am sure this is contributing to the sense of disengagement, the apathy, the view that politicians are “all the same” and can change nothing, and to the rise of extremist parties.”

He warns that the European “project has morphed and grown in such a way as to be unrecognisable” and makes clear that there is “nothing necessarily anti-European or xenophobic in wanting to vote Leave on June 23”.

Mr Johnson says that the influence of European courts on British life is “unstoppable and it is irreversible”.

“That is why EU law is likened to a ratchet, clicking only forwards,” he writes. “We are seeing a slow and invisible process of legal colonisation, as the EU infiltrates just about every area of public policy. Then – and this is the key point – the EU acquires supremacy in any field that it touches; because it is one of the planks of Britain’s membership, agreed in 1972, that any question involving the EU must go to Luxembourg, to be adjudicated by the European Court of Justice.”

Mr Johnson writes that the Prime Minister “has done his very best” over the course of his renegotiation and praises his bid to reassert the sovereignty of the British Parliament with a new Bill to be brought forward in the coming days.

However, he says the Prime Minister’s reforms “cannot stop the machine; at best it can put a temporary and occasional spoke in the ratchet”.

Calling for voters to be unafraid of a UK outside the EU, Mr Johnson writes: “If the ‘Leave’ side wins, it will indeed be necessary to negotiate a large number of trade deals at great speed. But why should that be impossible? We have become so used to Nanny in Brussels that we have become infantilised, incapable of imagining an independent future.”

Allies of the Prime Minister were last night furious at Mr Johnson’s decision to back the “Leave” campaign.

They accused him of making the announcement as part of a “calculated leadership bid” – an accusation that Mr Johnson yesterday rejected.

According to well-placed sources, Mr Johnson had given the Prime Minister his personal assurance just weeks ago that he intended to back the “In” campaign.

It is now expected that following Mr Johnson’s declaration, wealthy donors will swing behind the “Leave” campaign and pour resources into the bid to take Britain out of the EU.

Mr Johnson yesterday said he will not play a prominent role in the campaign and will not debate against Conservatives backing the “In” campaign.

Lord Heseltine, the former deputy prime minister, last night released a statement attacking Mr Johnson.

He said: “If he were to be successful in his ambition to cut us off from Europe, the flag would fly in Frankfurt and Paris in his honour.

“At a stroke, he would have blown away the safeguards for our financial services industry that the Prime Minister has just secured.

“That is to risk countless jobs across our country from Edinburgh in the north to Bournemouth in the south and, of course, London itself.”

The statement was released by the Britain Stronger in Europe campaign and echoed Mr Cameron’s claims that a “Brexit” would be a “leap in the dark”, risking accusations of a smear campaign against the Mr Johnson.